The invention relates to devices for detecting the presence of a substance having predetermined characteristics.
There has been a rapid growth in the use of carbon fiber composites, such as carbon fibers in an epoxy matrix, in civil aircraft markets and vehicular transportation systems. Recently, a significant hazard has been recognized that could ultimately prevent the widespread use of such materials even though they have desirable characteristics of high strength and stiffness relative to weight. This hazard occurs as a result of fires in which the epoxy matrix is consumed. The carbon fibers, which are electrically conductive, are neither oxidized or vaporized at the temperatures experienced during typical fires, and are expelled from the matrix and entrained in air to form aerosols which can travel considerable distances. The fibers can invade or settle in unprotected electrical or electronic equipment and, being conductive, can cause shorting, power failure and even blackouts. Automobile fires are common events and aircraft fires occur frequently. Fires in which carbon fiber composites are involved are often near airports, industrial or residential areas where shorting of electrical or electronic equipment could have life-endangering consequences.
Efforts are under way to overcome the recognized electrical hazards created by the burning of carbon fiber composites. The ultimate goal is to prevent release of fiber fragments from composites in a fire situation without sacrificing or compromising the proven good features of such composites. Several approaches have been and are currently being investigated in connection with such efforts. These approaches include gasifying the fibers, and clumping, retaining or insulating them to eliminate electrical conductivity. Essential to the success of these efforts is some quantitative test which provides an objective determination of the number of carbon fibers released during a fire. That is, some test is needed for quantifying the fiber release characteristics of modified composite materials in comparison with other composite materials. Several techniques are known in the prior art, all of which have certain disadvantages. A brass ball technique has been utilized in which the ball is charged by a high voltage power supply to about 1-2 kilovolts through a high impedance limiting resistor connected in parallel to a small capacitor. The charged ball thus attracts the fiber fragments in a flow stream. Upon contact with each fiber, the capacitance of the ball is changed, thus altering the characteristics of an electrical signal. The altered characteristics are correlatable to fiber length. Disadvantages of the technique are that the resultant signal is weak and amplification by sophisticated electronics is needed, the resultant signal is affected by strong electrical noise, fiber capture efficiency is low and dependent upon the flow velocity of the air containing the fibers, and the results are sensitive to moisture and soot in the flow stream. A low voltage grid system has also been utilized to detect the airborne fibers. This system utilizes parallel brass rods to form a grid supported by an insulation frame. The adjacent rods are biased by a low DC voltage typically around 15 volts. The fibers cause an electrical current to flow as they are collected by the grid, thereby producing an electrical signal related to fiber concentration. Problems with this system include loss of sensitivity due to fiber accumulation on the grid, a low count efficiency due to fibers passing through the grid, and ambiguities due to different contact resistances between the various fibers and the grid. Another conventional fiber detection system utilizes an optical scanner having a light source such as an LED or laser beam which is incident on a photodiode device. When air containing the fibers is passing between the light source and the photodiode device, the shadow of the fibers creates an electrical signal. A difficulty with the system is that it cannot differentiate fiber from soot or dust particles in the air. Another conventional system irradiates the airborne fibers by a laser beam, and measures backscattering from the fibers. This system is also subject to inaccuracies created by soot in the air. The present invention solves most of the disadvantages of the above systems by providing a means for detecting the presence of a fiber having predetermined dimensional characteristics.